The invention relates to a device for locking a set of gimbal-mounted detectors, and more particularly to a device for locking a set of three gimbal-mounted detectors in a logging sonde suitable for being lowered down a borehole. The invention is particularly applicable to geophones for detecting seismic waves along three orthogonal axes including a vertical axis, regardless of the orientation of the sonde. For purposes of clarity, the term "gimbal-mounted" as used herein shall refer to a mounted device having the ability to rotate with either one or two degrees of freedom.
Numerous types of devices exist for acquiring seismic signals. Particularly relevant is the sonde described in the commonly owned U.S. Pat. No. 4,563,757 to Decorps et al. When such a logging sonde is used in a borehole to record seismic waves generated by a distant source, it is preferable to detect the waves by means of a set of three geophones mounted in a three-axis arrangement. In this manner, one is able to evaluate the angle of incidence and the magnitude of the detected waves, and thereby expedite a more complete evaluation of the vertical seismic profile.
Most of the geophones available in the art have mechanical construction characteristics which require them to be placed at particular angles of inclination relative to the vertical (in the present application, vertical meaning the direction taken by a plumb line) in order to obtain optimum detection. For example, the longitudinal axis of the geophones may have to be located at 0 degrees (vertical) or at 90 degrees (horizontal) from the vertical. However, many boreholes deviate from the vertical, which has the direct consequence of causing the detection peformed by a set of geophones which is fixed relative to the sonde to be sub-optimal, and in some cases to be impossible.
In order to take account of these characteristics and thus operate under the best possible conditions, the solution proposed by the instant invention consists in detecting seismic signals along three orthogonal axes including the vertical, and to perform such detection regardless of the borehole deviation. Such an arrangement thus requires one geophone with vertical optimum detection and two geophones with horizontal optimum detection. As described in detail hereinafter, in order to provide for such an arrangement, use may be made of a gimbal mounting between the body of the sonde and a frame housing the three geophones. Systems using a gimbal mounting are known in the logging arts. For example, a system using a gimbal mounting for a dipmetering device is described in commonly owned U.S. Pat. No. 4,453,219 to Clavier et al. With reference to FIG. 2 of the Clavier et al. patent, the sonde contains a frame free to rotate about its longitudinal axis, said frame being equipped with an unbalanced mass and a pendulum. The pendulum is coupled to a potentiometer which measures the tilt of the sonde. Such a system thus seems to be at least partially adapted to performing the desired kinematic function. However, when applied to a set of geophones it is not completely free from drawbacks.